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SubjectRe: sys_epoll_wait high CPU load in 2.6.37
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Le mardi 25 janvier 2011 à 16:09 -0800, Simon Kirby a écrit :
> Hello!
>
> Since upgrading 2.6.36 -> 2.6.37, dovecot's "anvil" process seems to end
> up taking a lot more time in "top", and "perf top" shows output like this
> (system-wide):
>
> samples pcnt function DSO
> _______ _____ _____________________________ __________________________
>
> 2405.00 68.8% sys_epoll_wait [kernel.kallsyms]
> 33.00 0.9% mail_cache_lookup_iter_next libdovecot-storage.so.0.0.0
> 30.00 0.9% _raw_spin_lock [kernel.kallsyms]
> ...etc...
>
> It only wakes up 5-10 times per second or so (on this box), and does
> stuff like this:
>
> epoll_wait(12, {{EPOLLIN, {u32=19417616, u64=19417616}}}, 25, 2147483647) = 1
> read(29, "PENALTY-GET\t192.168.31.10\n"..., 738) = 26
> write(29, "0 0\n"..., 4) = 4
> epoll_wait(12, {{EPOLLIN, {u32=19395632, u64=19395632}}}, 25, 2147483647) = 1
> read(18, "LOOKUP\tpop3/192.168.31.10/tshield"..., 668) = 58
> write(18, "0\n"..., 2) = 2
> epoll_wait(12, {{EPOLLIN, {u32=19373072, u64=19373072}}}, 25, 2147483647) = 1
> read(7, "CONNECT\t3490\tpop3/192.168.31.10/t"..., 254) = 64
> epoll_wait(12, {{EPOLLIN, {u32=19373072, u64=19373072}}}, 25, 2147483647) = 1
> read(7, "DISCONNECT\t3482\tpop3/192.168.31.1"..., 190) = 62
>
> Anything obvious here? anvil talks over UNIX sockets to the rest of
> dovecot, and uses epoll_wait. So, suspect commits might be:
>
> 95aac7b1cd224f568fb83937044cd303ff11b029
> 5456f09aaf88731e16dbcea7522cb330b6846415
> or other bits from
> git log v2.6.36..v2.6.37 net/unix/af_unix.c fs/eventpoll.c
>
> I suspect it has something to do with that "infinite value" check removal
> in that first commit. It doesn't show up easily on a test box, but I can
> try reverting 95aac7b1cd in production if it's not obvious.
>
> Simon-

Yes, 95aac7b1cd is the problem, but anvil should use a 0 (no) timeout
instead of 2147483647 ms : epoll_wait() doesnt have to arm a timer in
this case, it is a bit faster.



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